1. Field of the Invention
Three-dimensional (3D) imaging, 3D-image projection, holography, color holography, electronic holography, digital holography, optical scanning holography, multicolor electronic holography, true-color 3D imaging and spatial light modulators.
2. Description of Prior Art
For many years holography has been investigated as a way to project three-dimensional (3D) information. Realistic 3D image projection has many applications both in the commercial and military sectors, such as entertainment, advertising, computer gaming for commercial markets, simulation, immersive training and 3D displays for military applications. For instance, army researcher have recently been investigating the use of photo-refractive crystals for holographic 3D displays. For example see xe2x80x9cThree-dimensional image reconstruction using strontium-barium-niobatexe2x80x9d by Brian P. Ketchel, Gary L. Wood, Richard J. Anderson and Gregory J. Salamo published in Applied Physics Letters, Vol. 71, p. 7-9, (1997) or xe2x80x9cThree-dimensional holographic display using a photo-refractive crystalxe2x80x9d by Christy A. Heid, Brian P. Ketchel, Gary L. Wood, Richard J. Anderson and Gregory J. Salamo, published in SPIE Vol. 3358, Proceedings of the Sixth International Symposium on Display Holography, p. 357-366, (1997). To date, there has been only limited success in applying holography techniques to the projection of realistic images, due largely to the single color nature of most hologram-reconstructed images. True-color holography (recording and reconstruction) has been investigated in detail and demonstrated using traditional photographic techniques, see for example, Practical Holography by Graham Saxby, published by Prentice Hall, New York, (1994) or xe2x80x9cColor-reflection holography: theory and experiment,xe2x80x9d by Paul M. Hubel and Laszlo Solymar published in Applied Optics, Vol. 30, No. 29, p. 4190-4203. (1991). In recent years, researchers have proposed electronic (or digital) holography techniques, by which the time consuming chemical processing involved in traditional holography is eliminated, see for example xe2x80x9cOptical Scanning Holography,xe2x80x9d by T. C. Poon, M. Wu, K. Shinoda, and Y. Suzuki, published in Proceedings of the IEEE, Vol. 84, p. 753-764 (1996). In electronic holography, holograms are recorded directly by photosensitive devices such as photodiodes or a CCD camera and usually stored as digital images, for example see xe2x80x9cDigital Recording and numerical reconstruction of holograms: reduction of the spatial frequency spectrum,xe2x80x9d by Ulf Schnars, Thomas M. Kreis, and Werner P. O. Juptner published in Optical Engineering, Vol. 35 No.4, p. 977-982, (1996) or xe2x80x9cThree-dimensional microscopy with phase-shifting digital holography,xe2x80x9d by Tong Zhang and lchirou Yamaguchi published in Optics Letters, Vol. 23, p. 1221-1223, (1998). These techniques have opened the way for such things as 3D holographic imaging in real-time, as indicated in xe2x80x9cReal-time Optical Holography Using a Spatial Light Modulatorxe2x80x9d by T. C. Poon, B. D. Duncan, M. H. Wu, K. Shinoda and Y. Suzuki published in the Japanese Journal of Applied Physics, Vol. 29, pp. Ll840-Ll842, (1990) and also in xe2x80x9cReal-Time Two-Dimensional Holographic Imaging Using an Electron-Beam-Addressed Spatial Light Modulator,xe2x80x9d by T. C. Poon, B. W. Schilling, M. H. Wu, K. Shinoda and Y. Suzuki published in Optics Letters, Vol. 18, pp. 63-65, (1993).
They have also opened the way to numerical image reconstruction as discussed in xe2x80x9cReal-time preprocessing of holographic information,xe2x80x9d by B. W. Schilling and T. C. Poon published in Optical Engineering, Vol. 34, No. 11, pp. 3174-3180. Nov. (1995). Just as digital cameras will supplant photographic cameras in many situations. Advances in electro-optic devices such as CCD cameras and spatial light modulators (SLMs) are making electronic holography not only a reality, but also the preferred method for 3D holographic imaging for many applications. It is notable that electronic 3D color display of computer generated holograms (CGH) has also been investigated and demonstrated as indicated in xe2x80x9cColor Images with the MIT Holographic Video Display,xe2x80x9d by Pierre St-Hilaire, Stephen A. Benton, Mark Lucente, and Paul M. Hubel published in SPIE Proc, Vol. 1667 Practical Holography Vl, p. 73-84, (1992) as well as in xe2x80x9cApproach to the Multicolor Imaging From Computer Generated Hologram,xe2x80x9d by Tadashi Nakamura, Hideya Takahashi, and Eiji Shimizu published in SPIE Proc, Vol. 2176 Practical Holography VIII, p. 102-107, (1994). However, these techniques offer no means to record a hologram of a real object, as holograms are generated via computer.
It is easy to see the many advantages digital holography has over traditional, photographic holography, and there are even more such advantages when applied to the true-color holography problem. In the past, true color holography has meant the recording of three separate monochromatic holograms, each at a different laser wavelength, superimposed on the same photographic plate as can be seen in the article xe2x80x9cColor-reflection holography: theory and experiment, mentioned above. Although the three separate holograms can be recorded simultaneously, often they are recorded in succession, sometimes using different emulsions and different exposure times. Saxby, above, cites a number of problematic practical considerations faced by true-color holographers, for instance the availability of truly panchromatic holographic emulsions, loss of fringe contrast, and cross talk. The lack of appropriate holographic emulsions forces the use of separate emulsions and therefore successive hologram recording for each color. For single emulsion holograms, fringe contrast can suffer due to very similar fringe patterns (from each color) occupying the same space in the photographic emulsion. Also, cross talk is a factor upon image reconstruction since the red laser will not only reconstruct the geometrically correct image from the xe2x80x9credxe2x80x9d fringes, but also two smaller, displaced images from the fringes created by the green and blue lasers.
FIG. 1 shows a system used for Optical Scanning Holography (OSH). Holographic recording by OSH is a technique, described in U.S. Pat. No. 5,064,257 for an xe2x80x9cOptical Heterodyne Scanning Type Holographic Devicexe2x80x9d which is based on scanning the object with a Fresnel Zone Pattern (FZP). The standard setup for mono-color holographic recording by OSH requires a two-beam laser light generator, which is a part of the system shown in FIG. 1. The two beams originate from the same laser 1 operating at frequency xcfx891. The original laser beam is separated into first and second beams using a beam-splitter (BSI) 2, that reflects nominally 50% of the original beam along an axis normal to the axis of the original laser beam. The first beam is passed through an Acousto-Optical Modulator 3 (AOM1) operating in the Bragg regime. Like other modulators this one produces sidebands of different orders of frequency, however, they are emitted at different angles. Here it is modulated with an electrically induced acoustic signal from an electrical signal generator 4 operating at cos (xcexa91t), e.g. xcexa91=40 MHz and the output angle is chosen to emit only the first order frequency (xcfx89+xcexa91)t. The second beam is further redirected by a mirror 7 as a third beam parallel to the first direct beam. This third beam passes through a beam expander 8 (BE2) to collimate the beam and increase its diameter to a predetermined size. The third beam also passes through a correction lens 9 to form a spherical wave. The modulated first beam (xcfx891+xcexa91)t is also passed through a beam expander 5 (BE1), similar to BE2 that collimates this first beam to have a plane wave-front and increases its diameter to match that of the third beam. The third and first beams define a co-dependent pair of output beams for one mono-color two-beam laser generator.
To create an FZP beam, the output beams of a co-dependent pair are redirected into a fourth or FZP beam, e.g. having an axis normally intersecting the axes of the pair, by reflectors 6 and 10, respectively. Reflector 6 may be a simple flat folding mirror that redirects the first beam so that its axis normally intersects the axis of the third beam. Reflector 10, however, must be a combining reflector, such as a second beam splitter or a narrow band filter mirror tuned to the frequency of the third beam. The latter passes at least about half of the redirected first beam and normally redirects an equal portion of the third beam onto the axis of the fourth beam. This combining reflector is centered on the axial intersection of the first and third generator beams. When the combining reflector is a beam-splitter each of the first and third beams emerge as first and second combined output beams, on the extended axes of the first and third beams, respectively. The splitter is designed to emit only the first combined beam. The axis of this beam thus becomes a shared main axis The unwanted second combined beam can be suppressed by placing a light absorbing layer 10a on the extended axis of the third beam to dissipate the unwanted second combined beam. The first combined output beam is next directed through a third beam-splitter 11 (BS3), to be described later, suppressing any reflected portion of this beam. The unreflected portion of this beam is next further redirected by an electric x-y scanning mirror 12 operating at standard horizontal and vertical frequencies; and finally arrives at a target 13. The target is thus immersed in a Fresnel Zone Pattern (FZP), which it reflects back to the scanning mirror producing a detection beam directed along the main shared axis and into the third beam-splitter, The normally reflected portion of the detection beam passes through lens 14 to be focused onto a detector axis. The unreflected portion of the return or detection beam is too weak and scattered to interact with the oppositely directed beams from the two-beam generator. The 3D location of each target scatterer is encoded in the position and size of the FZP, see for example xe2x80x9cFresnel Transformations of Images,xe2x80x9d by L. Mertz and N. O. Young published in the Proceedings of the conference on Optical Instruments and Techniques, K. J. Habell, ed. Chapman and Hall, London, pp. 305-312, (1962).
The intensity of the Fresnel Zone Pattem I (x, y, z, t), used for heterodyne detection, is mathematically represented by the following equation:                               I          ⁢                      xe2x80x83                    ⁢                      (                          x              ,              y              ,                              z                ;                t                                      )                          =                              A            2                    +                      B            2                    +                      2            ⁢            AB            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                                          (                                                                                                    k                        1                                                                    2                        ⁢                        z                                                              ⁡                                          [                                                                        x                          2                                                +                                                  y                          2                                                                    ]                                                        +                                                            Ω                      1                                        ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                            .                                                          (        1        )            
See, xe2x80x9cThree-dimensional microscopy by optical scanning holography,xe2x80x9d by T. C. Poon, K. B. Doh, B. W. Schilling, M. H. Wu, K. Shinoda, and Y. Suzuki published in Optical Engineering, Vol.34, No.5, pp. 1338-1344, (1995). In equation (1), A and B are the initial intensities of the light beams, k1=xcfx891/c is the wave-number of the laser light, c is the speed of light, and z is the axial distance measured from the focus of the spherical wave to the target. This interference pattern is raster-scanned over the object, which has an intensity reflectance given by |xcex93(x,y,z)|2. The target reflects the scanning light, which is collected by lens 14 (L4) and focused onto a photo detector 15, or for increased sensitivity, a photo-multiplier tube (PMT). The scanning action results in the spatial convolution of the scanning field intensity and the object, thus encoding each object scatterer by an FZP. Some insight into the OSH process may be gained by taking a simple example. Let the object transmittance be a single scatterer, |xcex93(x, y, z)|2=xcex4(x-x0, y-y0; z-z0). The resulting heterodyned output current from a band-pass filter 16. tuned to .xcexa91, (BPF1) is proportional to                                           i            Ω                    ⁢                      xe2x80x83                    ⁢                      (                          x              ,                              y                ;                z                            ,              t                        )                          ∝                  cos          ⁢                      xe2x80x83                    ⁢                                    (                                                                                          k                      1                                                              2                      ⁢                                              z                        0                                                                              [                                      xe2x80x83                                    ⁢                                                                                    (                                                  x                          -                                                      x                            0                                                                          )                                            2                                        +                                                                  (                                                  y                          -                                                      y                            0                                                                          )                                            2                                                        ]                                +                                                      Ω                    1                                    ⁢                                      xe2x80x83                                    ⁢                  t                                            )                        .                                              (        2        )            
This current contains the holographic information pertaining to the off-axis point object. In order to extract this information, the signal is electronically multiplied by cos (xcexa91t), produced by generator 18, in a conventional mixing circuit 17 commonly found in most audio and video receivers. This is fed to a low-pass filter 19. The resulting demodulated signal current id, which is represented by the following formula;                                           i            d                    ⁢                      xe2x80x83                    ⁢                      (                          x              ,                              y                ;                z                                      )                          ∝                  cos          ⁢                      xe2x80x83                    ⁢                                    (                                                                    k                    1                                                        2                    ⁢                                          z                      0                                                                      ⁡                                  [                                                                                    (                                                  x                          -                                                      x                            0                                                                          )                                            2                                        +                                                                  (                                                  y                          -                                                      y                            0                                                                          )                                            2                                                        ]                                            )                        .                                              (        3        )            
contains the location (x0, y0) as well as the depth (z0) information of the point object. In other words, the current id contains all the holographic information of the off-axis point source object. The intensity information obtained by demodulating current id may then be used to create a true hologram by synchronizing it with the x-y scanning signals and displaying the 2D image on a television type monitor, not shown. This electronic hologram can conveniently be sent to a video digitizer and converted to a digital image for storage and/or processing. The 3D object can then be reconstructed from the hologram either optically or digitally, as will be shown.
FIG. 2 shows the prior art method to achieve real image projection using either a photographic emulsion or other type of spatial light modulator (SLM). The three-dimensional image is reconstructed by illuminating the emulsion or SLM, containing the stored hologram, with the original reference beam used for recording. For photographic holography, this typically involves passing collimated laser light through the hologram. The collimated light is modulated by the interference pattern recorded in the hologram, and the real image is reconstructed. For digital or electronic holography, image reconstruction is often achieved numerically. However, for applications in which it is desired to project a real time image using a digital hologram, a high-resolution SLM can be used in a similar manner. Electronic SLMs have been investigated extensively for the purposes of hologram reconstruction, usually in conjunction with computer-generated holograms. Such reconstruction is shown in xe2x80x9cReal-time computer-generated hologram by means of liquid-crystal television spatial light modulatorxe2x80x9d by Fai Mok, Joseph Diep, Hua-Kuang Liu and Demetri Psaltis published in Optics Letters, Vol. 11, No. 11, p. 748-750 (1986) or xe2x80x9cComputer-generated holograms by means of a magneto optic spatial light modulatorxe2x80x9d by Joseph N. Mait and Glenn S. Himes published in Applied Optics, Vol. 28, No. 22, p. 4879-4887 (1989). Real-time hologram construction and reconstruction has been reported, however, using a high-resolution liquid-crystal SLM. For example an SLM with up to 175 line pairs/mm appears in xe2x80x9cReal-time Optical Holography Using a Spatial Light Modulatorxe2x80x9d cited above. Also as reported in the same article, applicants have experimented with an electron-beam-addressed spatial light modulator (EBSLM) and reconstructed real images recorded by the OSH technique in real-time. Suffice it to say that there are several types of commercially available SLMs, which may be used for coherent image reconstruction of this type. The details of real-image reconstruction using SLMs are case dependant and need not be addressed here. The situations are fundamentally the same, and the basic architecture is that shown in FIG. 2. The hologram is written to the SLM electronically, which in turn acts identically in the same way as the photographic hologram. The SLM (and hologram) is illuminated with the coherent light beam. The SLM then modulates the coherent light in accordance with the hologram pattern. The modulated light diffracts and, at some distance from the SLM, forms the real image stored in the hologram. It is fair to say that the use of an SLM to form a real-image reconstruction is well understood. Although not revolutionary, this technique of real-image projection is an integral part of the proposed true-color electronic holographic recording and 3D image projection system, according to the present invention.
A true-color three dimensional laser real-time image recording and projection system using active recording components such as, electronic heterodyne mixers, coherent primary color lasers, electro-optical detectors, electro-acousto-optic modulators; electro-optical spatial light modulators, combined with passive components including electronic band-pass filters; optical beam splitters, optical beam expanders, lenses and mirrors. Image projection preferably uses lasers, beam combiners and SLM""s.